Binance Auto-Subscribe?

Hey all,

this is probably a really stupid question, but I'm wondering about the "Auto-Subscribe"-option when staking a coin. It says that "the available balance of all Spot Accounts" will be used to "purchase flexible deposits".

I am mostly sure that this means that, e.g., all the BTC I have not staked will automatically be staked, but it also kinda sounds like "all available funds" (e.g., fiat and other coins) will be exchanged to BTC and then staked. Can someone please clarify? Thanks!